The Even-Odd Rule on Single Covalent-Bonded Structural Formulas as a Modification of Classical Structural Formulas of Multiple-Bonded Ions and Molecules

Geoffroy Auvert

doi:10.4236/ojpc.2014.44020

Abstract

In organic chemistry, as defined by Abegg, Kossel, Lewis and Langmuir, compounds are normally represented using structural formulas called Lewis structures. In these structures, the octet rule is used to define the number of covalent bonds that each atom forms with its neighbors and multiple bonds are frequent. Lewis’ octet rule has unfortunately shown limitations very early when applied to non-organic compounds: most of them remain incompatible with the “rule of eight” and location of charges is uncertain. In an attempt to unify structural formulas of octet and non-octet molecules or single-charge ions, an even-odd rule was recently proposed, together with a procedure to locate charge precisely. This even-odd rule has introduced a charge-dependent effective-valence number calculated for each atom. With this number and the number of covalent bonds of each element, two even numbers are calculated. These numbers are both used to understand and draw structuralformulas of single-covalent-bonded compounds. In the present paper, a procedure is proposed to adjust structural formulas of compounds that are commonly represented with multiple bonds. In order to keep them compatible with the even-odd rule, they will be represented using only single covalent bonds. The procedure will then describe the consequences of bond simplification on charges locations. The newly obtained representations are compared to their conventional structural formulas, i.e. single-bond representation vs. multiple-bond structures. Throughout the comparison process, charges are precisely located and assigned to specific atoms. After discussion of particular cases of compounds, the paper finally concludes that a rule limiting representations of multiplecovalent bonds to single covalent bonds, seems to be suitable for numerous known compounds.

Highlights

In classical 2D structural formulas of ions and molecules, single bonds are represented by drawing one line between neighbor atoms, like in di-hydrogen [1]
Other molecules are represented with multiple bonds, i.e. several lines between connected atoms: For instance in di-nitrogen where nitrogen atoms are connected with three lines [2]
These different types of bonds have been in combination with the octet rule to represent molecules for about one century [3]-[5]

Summary

Introduction

In classical 2D structural formulas of ions and molecules, single bonds are represented by drawing one line between neighbor atoms, like in di-hydrogen [1]. Other molecules are represented with multiple bonds, i.e. several lines between connected atoms: For instance in di-nitrogen where nitrogen atoms are connected with three lines [2]. In an attempt to address both octet and non-octet molecules, a new rule, named the “even-odd” rule, was recently proposed and shown to apply to molecules like Li2 [8]. This rule was subsequently checked against ions involving only single-bonded interconnections between neighbors [9]. The even-odd rule seems to be generally capable of representing many single-bonded compounds and well-known octet or non-octet compounds [8] [9]

Objectives

Discussion

Conclusion